Remove internal function usage from MarkdownRenderer

[ajalt]

This was an exercise in Mordant's public API. Rather than working with Lines directly, the renderer now uses linear layouts and invoked styles. This didn't end up needing much change to the renderer, and I actually found it easier to reason about.

Major changes:

• Style resets
• A separate implementation of VerticalLayout rather than using Table. Not strictly necessary, but it was easy and should be a little faster.